Prior art references recognize that tunnel junctions (TJs) can be useful in a variety of semiconductor devices, including GaN-based devices. Typically, standard GaN epitaxial growth ends with a p-contact; therefore, typical GaN TJs are p-to-n TJs.
However, actually forming a TJ on a GaN device can be challenging. In particular, the preferred method for GaN device epitaxy is metalorganic chemical vapor deposition (MOCVD). However, MOCVD may not be ideally suited for GaN-based TJs. P-GaN grown by MOCVD is typically unactivated due to the presence of hydrogen during the growth, and needs an activation step. However, it is known that this activation is typically difficult or impossible when the p-layer is covered by an n-layer, as is the embodiment in a TJ, because it is difficult for hydrogen (which causes compensation in as-MOCVD-grown Mg-doped GaN) to out-diffuse through n-type GaN. Further, the high doping levels which are desirable for TJs (such as 1E20 or above) are difficult to achieve in MOCVD due to the existence of compensating defects (such as Si complexes or Mg complexes in n-type and p-type GaN, respectively).
On the other hand, TJs are more readily achieved in GaN grown by molecular beam epitaxy (MBE). Growth can occur in a hydrogen-free ambient, so that p-GaN is activated as-grown. Growth can occur far from thermodynamic equilibrium, thus enabling high doping levels without the formation of compensating defects found in MOCVD.
Grundmann, M., et al., “Multi-color light emitting diode using polarization-induced tunnel junctions.” Phys. stat sol c 4, 2830 (2007) Malinverni, M., et al., “InGaN based micro light emitting diodes featuring a buried GaN tunnel junction.” Appl. Phys. Lett 107, 051107 (2015) demonstrated an MBE-grown III-Nitride TJ, using polarization fields to help injection. demonstrated an MBE-grown III-Nitride TJ, using straight n- and p-GaN.
Based on this, it has been proposed [Young, E., et al, “Hybrid tunnel junction contacts to III-nitride light-emitting diodes.” Appl. Phys. Expr. 9, 022102 (2016)] to combine MOCVD and MBE to grow a GaN device: in the embodiment of an LED, the active layers are grown by MOCVD (yielding high efficiency) and the TJ is grown by MBE (either fully or in part). This leads to good device performance. However, MBE machines are expensive, difficult to scale to large wafer diameters and throughputs, and not always robust, and thus may not be well suited for industrial implementation.
What is needed is a method to grow a high-quality hybrid GaN TJ with a second growth method which is inexpensive, readily scalable to large wafer diameters and throughputs, and robust. The present invention fulfills this need, among others.